In recent years, a technique called “phase difference method” has been employed as a method for obtaining a tracking control signal from an optical disc on which information is recorded by projecting and depressing pits, such as a CD (Compact Disc) or a DVD (Digital Video Disc).
Japanese Published Patent Application No. Hei.2001-67690 discloses an example of such phase difference method.
Hereinafter, a conventional tracking error detection apparatus disclosed in Japanese Published Patent Application No. Hei.2001-67690 will be described with reference to FIG. 19.
FIG. 19 is a block diagram illustrating the construction of the conventional tracking error detection apparatus.
As shown in FIG. 19, the conventional tracking error detection apparatus is provided with a photodetector 101 having photoreceptor elements 101a, 101b, 101c, and 101d that receive a reflected light beam from a light spot, and outputting photo currents according to the amounts of light received by the respective photoreceptor elements; first to fourth current-to-voltage converter 102a to 102d for converting the photo currents outputted from the photodetector 101 into voltage signals; signal generators, i.e., first and second adders 103a and 103b, for generating two signal sequences whose phases change depending on a tracking error of the light spot, from the voltage signals obtained by the first to fourth current-to-voltage converter 102a to 102d; first and second analog-to-digital converters (ADC) 104a and 104b for obtaining first and second digital signal sequences from the two signal sequences; first and second interpolation filters 105a and 105b for subjecting the inputted digital signals to interpolation; first and second zero cross point detection circuits 106a and 106b for detecting zero cross points of the first and second digital signal sequences which are interpolated by the first and second interpolation filters 105a and 105b, respectively; a phase difference detection circuit 107 for detecting a phase difference between the zero cross point of the first digital signal sequence and the zero cross point of the second digital signal sequence; and a low-pass filter (LPF) 108 for subjecting a phase comparison signal outputted from the phase difference detection circuit 107 to band restriction to obtain a tracking error signal. The photodetector 101 comprises the four photodetector elements 101a, 101b, 101c, and 101d that are partitioned in a tangential direction and a perpendicular direction with respect to an information track that is recorded as an information pit line on the recording medium. Among the signals which are generated according to the amounts of light received by the respective photoreceptor elements and are outputted from the photodetector 101, the output signals from the photoreceptor elements positioned on a diagonal line are added by each of the first and second adders 103a and 103b, thereby generating two sequences of digital signals. Further, a zerocross point is a point where an inputted digital signal intersects a center level of the digital signal that is calculated from an average value or the like of the digital signal.
Next, the operation of the conventional tracking error detection apparatus will be described.
Initially, in the photodetector 101, the respective photoreceptor elements 101a, 101b, 101c, and 101d receive a reflected light beam from a light spot that is obtained by irradiating a track on an optical recording medium (not shown) with a light beam, and output photo currents according to the amounts of received light.
The photo currents outputted from the respective photoreceptor elements of the photodetector 101 are converted into voltage signals by the first to fourth current-to-voltage conversion circuits 102a, 102b, 102c, and 102d, and the first adder 103a adds the outputs of the first and third current-to-voltage circuits 102a and 102c while the second adder 103b adds the outputs of the second and fourth current-to-voltage circuits 102b and 102d. 
Then, the signals outputted from the first and second adders 103a and 103b are subjected to sampling by the first and second ADCs 104a and 104b to be converted into first and second digital signal sequences, respectively.
Then, the digital signals outputted from the first and second ADCs 104a and 104b are input to the interpolation filters 105a and 105b to obtain interpolation data between the sampling data of the digital signals. Thereafter, zerocross points at the rising edges or falling edges of the two interpolated data sequences are detected by the zerocross point detection circuits 106a and 106b, respectively. For example, as a method of interpolation, “Nyquist interpolation” may be employed. As a method of detecting zerocross points at the rising or falling edges of two data sequences, change points of signs (+→− or −→+) in the interpolated data sequences may be obtained.
In the phase error detection circuit 107, a distance between the zerocross points in the waveforms of the first and second signal sequences is obtained using information of the zerocross points outputted from the zerocross point detection circuits 106a and 106b, and a phase comparison signal is detected on the basis of the distance between the zerocross points, and finally, band restriction is carried out by the LPF 108 to generate a tracking error signal of a frequency band that is required for tracking servo control.
Next, the construction and operation of the conventional phase difference detection circuit 107 will be described in more detail with reference to FIGS. 20 and 21.
FIG. 20 is a block diagram illustrating the construction of the conventional phase error detection circuit 107.
In FIG. 20, the phase difference detection circuit 107 comprises a phase difference calculation unit 201, a pulse generation unit 202, and a data updation unit 203.
The phase difference calculation unit 201 calculates a distance between the zerocross points of the two sequences of digital signals on the basis of the zerocross information detected by the zerocross point detection circuits 106a and 106b, and successively outputs it as a result of phase comparison to the data updation unit 203.
The pulse generation unit 202 generates pulse signals each corresponding to one sampling clock at the zerocross positions in the respective data sequences to be used for phase comparison, and outputs a pulse signal that appears later at the point where phase comparison is carried out, as a phase comparison end pulse, between the generated pulse signals corresponding to the respective data sequences.
The data updation unit 203 updates the output data for every phase comparison end pulse outputted from the pulse generation unit 202, using the phase comparison results that are successively outputted from the phase difference calculation unit 201, and maintains the output level of the output data until the next phase comparison end pulse arrives.
FIG. 21 is a diagram for explaining the operation of the phase difference detection circuit 107. FIG. 21 shows, from top to bottom, a first signal sequence outputted from the first zerocross point detection circuit 106a (phase comparison input A), a second signal sequence outputted from the second zerocross point detection circuit 106b (phase comparison input B), a phase comparison end pulse outputted from the pulse generation unit 202, and a phase comparison output from the phase difference detection circuit 11.
With reference to the phase comparison inputs A and B shown in FIG. 21, ∘ indicates sampling data obtained by the first or second ADC 104a or 104b, Δ indicates interpolation data sequences obtained by the first or second interpolation filters 105a or 105b, and ● and ▴ indicate zerocross points obtained from the sampling data sequences and the interpolation data sequences. Further, the phase comparison signal shown in FIG. 21 is obtained with respect to a vicinity of a specific track, and it is obtained at the falling edges of the two data sequences whose phase difference should be obtained. Further, the number of interpolation data is 3 (n=3).
When the outputs from the zerocross point detection circuits 106a and 106b are input to the phase difference detection circuit 107, the phase difference calculation unit 201 calculates a distance between the zerocross points detected by the zerocross point detection circuits 106a and 106b. Then, the pulse generation unit 202 generates a pulse signal corresponding to one sampling clock at a position where each of the data sequences (the phase comparison inputs A and B) to be used for phase comparison performs zerocross, and outputs a pulse signal that appears later between the generated pulse signals corresponding to the respective data sequences, as a phase comparison end pulse (refer to the phase comparison end pulse shown in FIG. 21).
Then, the data updation unit 203 performs updation of the output data using the phase comparison result outputted from the phase difference calculation unit 201, for every phase comparison end pulse outputted from the pulse generator 202, and maintains the output level of the output data until the next phase comparison end pulse arrives (refer to the phase comparison output shown in FIG. 21).
Thereby, the phase difference detection circuit 107 detects a phase comparison signal as shown by the phase comparison output in FIG. 21, and the tracking error signal obtained by performing band restriction to the phase comparison signal becomes an approximately straight signal when paying attention to a vicinity of a specific track. Then, the tracking error signal is observed over plural tracks, thereby obtaining, as a whole, an approximately sinusoidal waveform that is repeated for every track as shown in FIG. 22.
As described above, since the conventional tracking error detection apparatus can detect a tracking error by digital signal processing, it can deal with speedup of an optical recording/playback apparatus and an increase in recording density on a recording medium, which cannot be achieved by tracking error detection using analog signal processing. Furthermore, the constituents relating to analog signal processing can be significantly reduced, thereby realizing small-sized and low-cost optical recording/playback apparatus.
In the above-mentioned conventional tracking error detection apparatus, however, since AD conversion by the first and second ADCs 104a and 104b is carried out with the sampling rate being fixed, the amplitude of the obtained tracking error signal varies between the inner track and the outer track of the disc during CAV playback.
That is, since the channel rate is low at the inner track while it is high at the outer track during CAV playback, when the first and second ADCs 104a and 104b performs AD conversion with the sampling rate being fixed, the number of sampling data to be sampled within the same phase interval becomes larger at the inner track than at the outer track, resulting in a variation in the amplitudes of the tracking error signals obtained at the inner track and the outer track on the disc.
FIGS. 23(a) and 23(b) show tracking error signals detected by the conventional tracking error detection apparatus during CAV playback. To be specific, FIG. 23(a) shows a tracking error signal at the inner track of the disc while FIG. 23(b) shows a tracking error signal at the outer track of the disc.
As shown in FIG. 23(a), at the inner track, the number of sampling data to be sampled within the same phase interval increases, and therefore, the phase difference detected as the distance between the zerocross points by the phase difference detection circuit 107 increases, resulting in an increase in the output amplitude of the tracking error signal. On the other hand, at the outer track, as shown in FIG. 23(b), the number of sampling data to be sampled within the same phase interval is small, and therefore, the phase difference detected as the distance between the zerocross points by the phase difference detection circuit 107 becomes small, resulting in a reduction in the output amplitude of the tracking error signal.
Further, in the above-mentioned conventional tracking error detection apparatus, in order to realize a small-size and low-cost optical recording/playback apparatus, bit resolution of the first and second ADCs 104a and 104b possessed by the conventional tracking error detection apparatus is set at a minimum bit resolution required for phase comparison. Therefore, when the amplitude of the analog signal to be input to the first and second ADCs 104a and 104b is not sufficiently obtained due to defect or the like, sampling is not correctly carried out by the first and second ADCs 104a and 104b, leading to false detection of the phase difference detection circuit 107.
Further, in the conventional tracking error detection apparatus, when the voltage level of the analog signal to be input to the first and second ADCs 104a and 104b varies due to defect or the like, the zerocross point detection circuits 106a and 106b cannot correctly detect zerocross points, and the phase difference detection circuit 107 cannot detect a phase difference.